Federal agents raided the Mingei International Museum yesterday. Authorities said about 70 items there probably were illegally obtained.

The Mingei International Museum in Balboa Park is one of five Southern California museums caught up in a federal investigation involving stolen ancient Thai artifacts, a so-called grave robber and tax fraud.

Court documents used to obtain search warrants detail an elaborate scheme that reads like a whodunit.

The five-year probe culminated yesterday when agents searched four of the five museums and two Los Angeles-area art galleries. No one has been criminally charged.

Along with the Mingei, the investigation focused on the Los Angeles County Museum of Art, the Pacific Asia Museum in Pasadena, the Bowers Museum in Santa Ana and the UC Berkeley Art Museum.

All are suspected of some degree of fraud related to the scheme, said a federal source who spoke on condition of anonymity because the investigation is ongoing.

Although court documents refer to the Mingei museum as “a repository for stolen Thai archaeological resources,” museum director Rob Sidner said neither he nor his staff knew artifacts received by the Mingei might have been smuggled into the United States.

“We believe we did absolute due diligence in these cases,” said Sidner, who has worked at the museum for 15 years, the past two as director. He said any museum director would be a fool to accept stolen objects.

“Our museum has a stellar reputation for its honesty and being aboveboard,” Sidner said yesterday at an impromptu news conference in front of the closed museum. “We’re an open book, and this is horribly shocking to us.”

The court records don’t specifically accuse museum staff members, but the affidavit that deals with the Mingei suggests the scams should have been easy to spot.

Sidner and Terri Bryson, the Mingei’s registrar, are named in the affidavit, which links the scams to a single smuggling suspect, Bob Olson of Los Angeles, and Los Angeles art dealer Jon Markell, who owns the Silk Roads gallery with his wife, Cari.

Olson, who is referred to as a “grave robber” in the court documents, is accused of stealing most of the items from northeastern Thailand.

Olson couldn’t be located for comment, and Markell didn’t return phone calls.

The scam detailed in the Mingei affidavit played out like this: Olson and Markell sold stolen Thai artifacts to an undercover agent with the National Park Service who posed as an artifacts dealer, arranging for the agent to donate the items to the Mingei in exchange for a tax deduction.

The items were routinely overvalued by appraisers.

A 10,000-year-old stone ax head, for example, is worth $150 but could be donated to certain museums at an appraised value of $1,000 or more, the Mingei affidavit says.

But some donations were routinely valued at less than $5,000 so that an appraisal to support the tax deduction wasn’t required.

The court records list several examples of donations valued at just under $5,000, including at least three from the undercover agent to the Mingei: $4,985 in June 2006, $4,900 in March 2007 and $4,915 later that month.

Federal authorities said about 70 items at the Mingei probably were illegally obtained. No estimates were given for the other museums.

The affidavit says the undercover agent made Sidner and his curators aware of a Thai law that prohibits exporting antiques without a license, giving them copies of the statute in 2006. Sidner said yesterday that he never saw the document.

“We probably should have known, but because of the bona fides of the people who we have relied on and have been our advisers over the years, we have been assured these were absolutely proper,” Sidner said.

He also said the Markells, the gallery owners, were “vouched for” by people whom museum officials trusted. “We wouldn’t deal with anybody who we didn’t think was absolutely first-rate,” he said.

Martha Longenecker, the Mingei’s founder, who led the museum until late 2005, was taken aback by the developments.

“That the museum would be involved in anything intentionally illegal is ridiculous,” Longenecker said.

Authorities made more specific allegations in the cases involving the Bowers Museum and the Pacific Asia Museum.

An affidavit in the Bowers case recounts a 2004 conversation between Bowers chief curator Armand Labbe, now deceased, and an undercover agent.

“Labbe also commented that he was torn because on one hand he wanted to abide by the law, and on the other hand, he wanted the collections,” the affidavit says.

The funneled loot suspected of landing at the Mingei comes from the Ban Chiang culture, which existed from 1000 B.C. to about 200 A.D. in northeastern Thailand.

The first Ban Chiang object had come into the collection courtesy of Labbe, who was a supporter of the Mingei. Labbe introduced Longenecker and Sidner to the Markells, who facilitated other donations of Ban Chiang works.

The original location of the Ban Chiang culture was named a World Heritage Site in 1992 and is considered the most important prehistoric settlement in Southeast Asia, the court documents say.

Agents arrived at the Mingei just before 8 a.m. yesterday and didn’t leave until after 6 p.m. Items were fingerprinted and tagged, and piles of documents were seized, but no artifacts were removed.

Museum officials expected to reopen today.

Whatever transpires with the continuing investigation, it’s clear that it already has affected how the museum will add to its collection.

“Even if we deeply trust the people who come to us, if we’re not absolutely clear on the origins of the piece, we’ll have to think twice about taking it,” Sidner said. “We’ll put more structures in place to check them. There’s no question that this sort of thing affects the art world adversely.”

The Mingei is an unusual folk-art museum founded 30 years ago by Longenecker, a San Diego ceramic artist. The word “mingei” comes from the Japanese words for people, min, and art, gei. The museum displays “arts of daily use,” which might be an ornate Mexican plate made by a long-dead craftsman or Japanese origami by a contemporary designer.

The museum started collecting Thai objects in 1998. It has amassed dozens of artifacts, including pottery, bronze bangles and bracelets, and shell and stone pieces, Sidner said.

Museum officials had expected to mount an exhibit, to be called “Ban Chiang, Art of Ancient Thailand,” on March 1.

Now, with many of those pieces under investigation, the planned exhibit is on hold.